Network Time Security for the Network Time
ProtocolAkamai Technologies150 BroadwayCambridgeMA02142United Statesdafranke@akamai.comhttps://www.dfranke.usPhysikalisch-Technische
BundesanstaltBundesallee 100BraunschweigD-38116Germany+49-(0)531-592-8420+49-531-592-698420dieter.sibold@ptb.dePhysikalisch-Technische
BundesanstaltBundesallee 100BraunschweigD-38116Germany+49-(0)531-592-4471kristof.teichel@ptb.demarcus@dansarie.seNetnodragge@netnod.seInternet Area
NTP Working GroupIntegrityAuthenticationNTPSecurity
This memo specifies Network Time Security (NTS), a mechanism
for using Transport Layer Security (TLS) and Authenticated
Encryption with Associated Data (AEAD) to provide
cryptographic security for the client-server mode of the
Network Time Protocol (NTP).
NTS is structured as a suite of two loosely coupled sub-protocols.
The first (NTS-KE) handles initial authentication and key
establishment over TLS. The second handles encryption and
authentication during NTP time via extension fields in the
NTP packets, and holds all required state only on the
client via opaque cookies.
This memo specifies Network Time Security (NTS), a
cryptographic security mechanism for network time
synchronization. A complete specification is provided for
application of NTS to the client-server mode of the
Network Time Protocol (NTP).
The objectives of NTS are as follows:
Identity: Through the use of the X.509 public key infrastructure,
implementations may cryptographically establish the identity of
the parties they are communicating with.
Authentication: Implementations may cryptographically
verify that any time synchronization packets are
authentic, i.e., that they were produced by an
identified party and have not been modified in transit.
Confidentiality: Although basic time synchronization
data is considered non-confidential and sent in the
clear, NTS includes support for encrypting NTP extension
fields.
Replay prevention: Client implementations may detect when
a received time synchronization packet is a replay of
a previous packet.
Request-response consistency: Client implementations may
verify that a time synchronization packet received from
a server was sent in response to a particular request from
the client.
Unlinkability: For mobile clients, NTS will not leak any
information additional to NTP which would permit a
passive adversary to determine that two packets sent
over different networks came from the same client.
Non-amplification: Implementations (especially server
implementations) may avoid acting as distributed
denial-of-service (DDoS) amplifiers by never responding to a
request with a packet larger than the request packet.
Scalability: Server implementations may serve large
numbers of clients without having to retain any
client-specific state.
The Network Time Protocol includes many different operating modes to
support various network topologies. In addition to its best-known and
most-widely-used client-server mode, it also includes modes for
synchronization between symmetric peers, a control mode for server
monitoring and administration, and a broadcast mode. These various
modes have differing and partly contradictory requirements for
security and performance. Symmetric and control modes demand mutual
authentication and mutual replay protection. Additionally, for certain
message types control mode may require confidentiality as well as
authentication. Client-server mode places more stringent requirements
on resource utilization than other modes, because servers may have
vast number of clients and be unable to afford to maintain per-client
state. However, client-server mode also has more relaxed security
needs, because only the client requires replay protection: it is
harmless for stateless servers to process replayed packets. The
security demands of symmetric and control modes, on the other hand,
are in conflict with the resource-utilization demands of client-server
mode: any scheme which provides replay protection inherently involves
maintaining some state to keep track of what messages have already
been seen.
This memo specifies NTS exclusively for the client-server mode of NTP.
To this end, NTS is structured as a suite of two protocols:
The "NTS Extensions for NTPv4" define a collection of NTP
extension fields for cryptographically securing NTPv4 using
previously-established key material. They are suitable for
securing client-server mode because the server can implement them
without retaining per-client state. All state is kept by the
client and provided to the server in the form of an encrypted
cookie supplied with each request. On the other hand, the NTS
Extension Fields are suitable *only* for client-server mode
because only the client, and not the server, is protected from
replay.
The "NTS Key Establishment" protocol (NTS-KE) is a
mechanism for establishing key material for use with the NTS
Extension Fields for NTPv4. It uses TLS to exchange keys, provide
the client with an initial supply of cookies, and negotiate some
additional protocol options. After this exchange, the TLS channel
is closed with no per-client state remaining on the server side.
The typical protocol flow is as follows: The client connects to an
NTS-KE server on the NTS TCP port and the two parties perform a TLS
handshake. Via the TLS channel, the parties negotiate some additional
protocol parameters and the server sends the client a supply of
cookies along with a list of one or more IP addresses to NTP servers
for which the cookies are valid. The parties use
TLS key export to extract key material
which will be used in the next phase of the protocol. This negotiation
takes only a single round trip, after which the server closes the
connection and discards all associated state. At this point the NTS-KE
phase of the protocol is complete. Ideally, the client never needs to
connect to the NTS-KE server again.
Time synchronization proceeds with one of the indicated NTP servers
over the NTP UDP port. The client sends the server an NTP client
packet which includes several extension fields. Included among these
fields are a cookie (previously provided by the key exchange server)
and an authentication tag, computed using key material extracted from
the NTS-KE handshake. The NTP server uses the cookie to recover this
key material and send back an authenticated response. The response
includes a fresh, encrypted cookie which the client then sends back in
the clear in a subsequent request. (This constant refreshing of
cookies is necessary in order to achieve NTS's unlinkability goal.)
provides an overview of the
high-level interaction between the client, the NTS-KE server, and the
NTP server. Note that the cookies' data format and the exchange of
secrets between NTS-KE and NTP servers are not part of this
specification and are implementation dependent. However, a suggested
format for NTS cookies is provided in
.
| NTP Server 1 |
| | |
Shared cookie | +--------------+
+---------------+ encryption parameters | +--------------+
| | (Implementation dependent) | | |
| NTS-KE Server | | NTP Server 2 |
| | | | |
+---------------+ | +--------------+
^ | .
| | .
| 1. Negotiate parameters, | .
| receive initial cookie | +--------------+
| supply, generate AEAD keys, | | |
| and receive NTP server IP +-> | NTP Server N |
| addresses using "NTS Key | |
| Establishment" protocol. +--------------+
| ^
| |
| +----------+ |
| | | |
+-----------> | Client |
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 when,
and only when, they appear in all capitals, as shown here.
Network Time Security makes use of TLS for NTS key establishment.
Since the NTS protocol is new as of this publication, no
backward-compatibility concerns exist to justify using
obsolete, insecure, or otherwise broken TLS features or
versions. Implementations MUST conform with or with a later revision of BCP
195. Furthermore:
Implementations MUST NOT negotiate TLS versions earlier than
1.2, SHOULD negotiate TLS 1.3 or
later when possible, and MAY refuse to negotiate any TLS
version which has been superseded by a later supported
version.
Use of the Application-Layer Protocol
Negotiation Extension is integral to NTS and support for
it is REQUIRED for interoperability.
The NTS key establishment protocol is conducted via TCP port [[TBD1]].
The two endpoints carry out a TLS handshake in conformance with
, with the client offering (via an
ALPN extension), and the server accepting,
an application-layer protocol of "ntske/1". Immediately
following a successful handshake, the client SHALL send a single request
as Application Data encapsulated in the TLS-protected channel. Then, the
server SHALL send a single response followed by a TLS
"Close notify" alert and then discard the channel state.
The client's request and the server's response each SHALL consist of a
sequence of records formatted according to
. Requests and non-error responses each
SHALL include exactly one NTS Next Protocol Negotiation record. The
sequence SHALL be terminated by a "End of Message" record. The
requirement that all NTS-KE messages be terminated by an End of Message
record makes them self-delimiting.
Clients and servers MAY enforce length limits on requests and responses,
however, servers MUST accept requests of at least 1024 octets and
clients SHOULD accept responses of at least 65536 octets.
The fields of an NTS-KE record are defined as follows:
C (Critical Bit): Determines the disposition of unrecognized Record
Types. Implementations which receive a record with an unrecognized
Record Type MUST ignore the record if the Critical Bit is 0 and MUST
treat it as an error if the Critical Bit is 1.
Record Type Number: A 15-bit integer in network byte order. The
semantics of record types 0–6 are specified in this memo.
Additional type numbers SHALL be tracked through the IANA Network
Time Security Key Establishment Record Types registry.
Body Length: The length of the Record Body field, in octets, as a
16-bit integer in network byte order. Record bodies MAY have any
representable length and need not be aligned to a word boundary.
Record Body: The syntax and semantics of this field SHALL be
determined by the Record Type.
For clarity regarding bit-endianness: the Critical Bit is the
most-significant bit of the first octet. In C, given a network buffer
`unsigned char b[]` containing an NTS-KE record, the critical bit is
`b[0] >> 7` while the record type is
`((b[0] & 0x7f) << 8) + b[1]`.
provides a schematic overview of the
key exchange. It displays the protocol steps to be performed by the NTS
client and server and record types to be exchanged.
|
| o End of Message |
+-----------------+---------------------+
|
|
Server -----------+---------------+-----+----------------------->
^ \
/ \
/ TLS application \
/ data \
/ \
/ V
Client -----+---------------------------------+---------------->
| |
| |
| |
+-----------+----------------------+ +------+-----------------+
|- Generate KE request message. | |- Verify server response|
| - Include Record Types: | | message. |
| o NTS Next Protocol Negotiation | |- Extract cookie(s). |
| o AEAD Algorithm Negotiation | | |
| o | | |
| o End of Message | | |
+----------------------------------+ +------------------------+]]>
The following NTS-KE Record Types are defined:
The End of Message record has a Record Type number of 0 and a
zero-length body. It MUST occur exactly once as the final record of
every NTS-KE request and response. The Critical Bit MUST be set.
The NTS Next Protocol Negotiation record has a Record Type number
of 1. It MUST occur exactly once in every NTS-KE request and
response. Its body consists of a sequence of 16-bit unsigned
integers in network byte order. Each integer represents a Protocol
ID from the IANA Network Time Security Next Protocols registry. The
Critical Bit MUST be set.
The Protocol IDs listed in the client's NTS Next Protocol
Negotiation record denote those protocols which the client wishes to
speak using the key material established through this NTS-KE
session. The Protocol IDs listed in the server's response MUST
comprise a subset of those listed in the request and denote those
protocols which the server is willing and able to speak using the
key material established through this NTS-KE session. The client MAY
proceed with one or more of them. The request MUST list at least one
protocol, but the response MAY be empty.
The Error record has a Record Type number of 2. Its body is exactly
two octets long, consisting of an unsigned 16-bit integer in network
byte order, denoting an error code. The Critical Bit MUST be set.
Clients MUST NOT include Error records in their request. If clients
receive a server response which includes an Error record, they MUST
discard any negotiated key material and MUST NOT proceed to the Next
Protocol.
The following error codes are defined:
Error code 0 means "Unrecognized Critical Record". The
server MUST respond with this error code if the request included
a record which the server did not understand and which had its
Critical Bit set. The client SHOULD NOT retry its request
without modification.
Error code 1 means "Bad Request". The server MUST
respond with this error if, upon the expiration of an
implementation-defined timeout, it has not yet received a
complete and syntactically well-formed request from the client.
The Warning record has a Record Type number of 3. Its body is
exactly two octets long, consisting of an unsigned 16-bit integer in
network byte order, denoting a warning code. The Critical Bit MUST
be set.
Clients MUST NOT include Warning records in their request. If
clients receive a server response which includes a Warning record,
they MAY discard any negotiated key material and abort without
proceeding to the Next Protocol. Unrecognized warning codes MUST be
treated as errors.
This memo defines no warning codes.
The AEAD Algorithm Negotiation record has a Record Type number of 4.
Its body consists of a sequence of unsigned 16-bit integers in
network byte order, denoting Numeric Identifiers from the IANA
AEAD registry. The Critical Bit MAY be
set.
If the NTS Next Protocol Negotiation record offers Protocol ID 0
(for NTPv4), then this record MUST be included exactly once. Other
protocols MAY require it as well.
When included in a request, this record denotes which AEAD
algorithms the client is willing to use to secure the Next Protocol,
in decreasing preference order. When included in a response, this
record denotes which algorithm the server chooses to use. It is
empty if the server supports none of the algorithms offered. In
requests, the list MUST include at least one algorithm. In
responses, it MUST include at most one. Honoring the client's
preference order is OPTIONAL: servers may select among any of the
client's offered choices, even if they are able to support some
other algorithm which the client prefers more.
Server implementations of NTS extension fields for
NTPv4 MUST support AEAD_AES_SIV_CMAC_256 (Numeric Identifier
15). That is, if the client includes AEAD_AES_SIV_CMAC_256 in its
AEAD Algorithm Negotiation record and the server accepts Protocol
ID 0 (NTPv4) in its NTS Next Protocol Negotiation record, then the
server's AEAD Algorithm Negotiation record MUST NOT be empty.
The New Cookie for NTPv4 record has a Record Type number of 5. The
contents of its body SHALL be implementation-defined and clients
MUST NOT attempt to interpret them. See for a suggested
construction.
Clients MUST NOT send records of this type. Servers MUST send at
least one record of this type, and SHOULD send eight of them, if the
Next Protocol Negotiation response record contains Protocol ID 0
(NTPv4) and the AEAD Algorithm Negotiation response record is not
empty. The Critical Bit SHOULD NOT be set.
The NTPv4 Server Negotiation record has a Record Type
number of 6. Its body consists of an ASCII-encoded string
conforming to the syntax of the Host subcomponent of a URI
(). IPv6 addresses MUST NOT
include zone identifiers .
When NTPv4 is negotiated as a Next Protocol and this
record is sent by the server, the body specifies the
hostname or IP address of the NTPv4 server with which the
client should associate and which will accept the supplied
cookies. If no record of this type is sent, the client
SHALL interpret this as a directive to associate with an
NTPv4 server at the same IP address as the NTS-KE server.
Servers MUST NOT send more than one record of this type.
When this record is sent by the client, it indicates that
the client wishes to associate with the specified NTP
server. The NTS-KE server MAY incorporate this request when
deciding what NTPv4 Server Negotiation records to respond
with, but honoring the client's preference is
OPTIONAL. The client MUST NOT send more than one record of
this type.
Servers MAY set the Critical Bit on records of this type;
clients SHOULD NOT.
The NTPv4 Port Negotiation record has a Record Type number
of 7. Its body consists of a 16-bit unsigned integer in
network byte order, denoting a UDP port number.
When NTPv4 is negotiated as a Next Protocol and this
record is sent by the server, the body specifies the port
number of the NTPv4 server with which the client should
associate and which will accept the supplied cookies. If
no record of this type is sent, the client SHALL assume
a default of 123 (the registered port number for NTP).
When this record is sent by the client in conjunction with
a NTPv4 Server Negotiation record, it indicates that the
client wishes to associate with the NTP server at the
specified port. The NTS-KE server MAY incorporate this
request when deciding what NTPv4 Server Negotiation and
NTPv4 Port Negotiation records to respond with, but
honoring the client's preference is OPTIONAL.
Servers MAY set the Critical Bit on records of this type;
clients SHOULD NOT.
Following a successful run of the NTS-KE protocol, key material SHALL
be extracted according to RFC 5705.
Inputs to the exporter function are to be constructed in a manner
specific to the negotiated Next Protocol. However, all protocols which
utilize NTS-KE MUST conform to the following two rules:
The disambiguating label string MUST be
"EXPORTER-network-time-security/1".
The per-association context value MUST be provided and MUST begin
with the two-octet Protocol ID which was negotiated as a Next
Protocol.
Following a successful run of the NTS-KE protocol wherein Protocol
ID 0 (NTPv4) is selected as a Next Protocol, two AEAD keys SHALL be
extracted: a client-to-server (C2S) key and a server-to-client (S2C)
key. These keys SHALL be computed according to RFC 5705, using the following inputs.
The disambiguating label string SHALL be
"EXPORTER-network-time-security/1".
The per-association context value SHALL consist of the following
five octets:
The first two octets SHALL be zero (the Protocol ID for
NTPv4).
The next two octets SHALL be the Numeric Identifier of the
negotiated AEAD Algorithm in network byte order.
The final octet SHALL be 0x00 for the C2S key and 0x01 for the
S2C key.
Implementations wishing to derive additional keys for private or
experimental use MUST NOT do so by extending the above-specified
syntax for per-association context values. Instead, they SHOULD use
their own disambiguating label string. Note that RFC 5705 provides that disambiguating label
strings beginning with "EXPERIMENTAL" MAY be used without
IANA registration.
In general, an NTS-protected NTPv4 packet consists of:
The usual 48-octet NTP header which is authenticated but not
encrypted.
Some extension fields which are authenticated but not encrypted.
An extension field which contains AEAD output (i.e., an
authentication tag and possible ciphertext). The corresponding
plaintext, if non-empty, consists of some extension fields which
benefit from both encryption and authentication.
Possibly, some additional extension fields which are neither
encrypted nor authenticated. These are discarded by the
receiver.
Always included among the authenticated or authenticated-and-encrypted
extension fields are a cookie extension field and a unique identifier
extension field. The purpose of the cookie extension field is to
enable the server to offload storage of session state onto the client.
The purpose of the unique identifier extension field is to protect the
client from replay attacks.
The Unique Identifier extension field provides the client with a
cryptographically strong means of detecting replayed packets. It has a
Field Type of [[TBD2]]. When the extension field is included in a
client packet (mode 3), its body SHALL consist of a string of octets
generated uniformly at random. The string MUST be at least 32 octets
long. When the extension field is included in a server packet
(mode 4), its body SHALL contain the same octet string as was provided
in the client packet to which the server is responding. All server
packets generated by NTS-implementing servers in response to client
packets containing this extension field MUST also contain this field
with the same content as in the client's request. The field's use in
modes other than client-server is not defined.
This extension field MAY also be used standalone, without NTS, in
which case it provides the client with a means of detecting spoofed
packets from off-path attackers. Historically, NTP's origin timestamp
field has played both these roles, but for cryptographic purposes this
is suboptimal because it is only 64 bits long and, depending on
implementation details, most of those bits may be predictable. In
contrast, the Unique Identifier extension field enables a degree of
unpredictability and collision resistance more consistent with
cryptographic best practice.
The NTS Cookie extension field has a Field Type of [[TBD3]]. Its
purpose is to carry information which enables the server to recompute
keys and other session state without having to store any per-client
state. The contents of its body SHALL be implementation-defined and
clients MUST NOT attempt to interpret them. See for a suggested
construction. The NTS Cookie extension field MUST NOT be included in
NTP packets whose mode is other than 3 (client) or 4 (server).
The NTS Cookie Placeholder extension field has a Field Type of
[[TBD4]]. When this extension field is included in a client packet
(mode 3), it communicates to the server that the client wishes it to
send additional cookies in its response. This extension field MUST NOT
be included in NTP packets whose mode is other than 3.
Whenever an NTS Cookie Placeholder extension field is present, it MUST
be accompanied by an NTS Cookie extension field. The body length of
the NTS Cookie Placeholder extension field MUST be the same as the
body length of the NTS Cookie extension field. This length requirement
serves to ensure that the response will not be larger than the
request, in order to improve timekeeping precision and prevent DDoS
amplification. The contents of the NTS Cookie Placeholder extension
field's body are undefined and, aside from checking its length, MUST
be ignored by the server.
The NTS Authenticator and Encrypted Extension Fields extension field
is the central cryptographic element of an NTS-protected NTP packet.
Its Field Type is [[TBD5]]. It SHALL be formatted according to
and include the following fields:
Nonce length: Two octets in network byte order, giving the length
of the Nonce field, excluding any padding, interpreted as an
unsigned integer.
Ciphertext Length: Two octets in network byte order, giving the
length of the Ciphertext field, excluding any padding, interpreted
as an unsigned integer.
Nonce: A nonce as required by the negotiated AEAD Algorithm. The
field is zero-padded to a word (four octets) boundary.
Ciphertext: The output of the negotiated AEAD Algorithm. The
structure of this field is determined by the negotiated algorithm,
but it typically contains an authentication tag in addition to the
actual ciphertext. The field is zero-padded to a word (four
octets) boundary.
Additional Padding: Clients which use a nonce length shorter than
the maximum allowed by the negotiated AEAD algorithm may be required
to include additional zero-padding. The necessary length of this
field is specified below.
The Ciphertext field SHALL be formed by providing the following inputs
to the negotiated AEAD Algorithm:
K: For packets sent from the client to the server, the C2S key
SHALL be used. For packets sent from the server to the client, the
S2C key SHALL be used.
A: The associated data SHALL consist of the portion of the NTP
packet beginning from the start of the NTP header and ending at
the end of the last extension field which precedes the NTS
Authenticator and Encrypted Extension Fields extension field.
P: The plaintext SHALL consist of all (if any) NTP extension
fields to be encrypted. The format of any such fields SHALL be in
accordance with RFC 7822. If
multiple extension fields are present they SHALL be joined by
concatenation.
N: The nonce SHALL be formed however required by the negotiated
AEAD algorithm.
The purpose of the Additional Padding field is to ensure
that servers can always choose a nonce whose length is
adequate to ensure its uniqueness, even if the client
chooses a shorter one, and still ensure that the overall
length of the server's response packet. does not exceed the
length of the request. For mode 4 (server) packets, no
Additional Padding field is ever required. For mode 3
(client) packets, the length of the Additional Padding field
SHALL be computed as follows. Let `N_LEN` be the padded
length of the the Nonce field. Let `N_MAX` be, as specified
by RFC 5116, the maximum
permitted nonce length for the negotiated AEAD
algorithm. Let `N_REQ` be the lesser of 16 and N_MAX,
rounded up to the nearest multiple of 4. If N_LEN is
greater than or equal to N_REQ, then no Additional Padding
field is required. Otherwise, the Additional Padding field
SHALL be at least N_REQ - N_LEN octets in length. Servers
MUST enforce this requirement by discarding any packet which
does not conform to it.
The NTS Authenticator and Encrypted Extension Fields extension field
MUST NOT be included in NTP packets whose mode is other than 3
(client) or 4 (server).
A client sending an NTS-protected request SHALL include the following
extension fields as displayed in :
Exactly one Unique Identifier extension field which MUST be
authenticated, MUST NOT be encrypted, and whose contents MUST NOT
duplicate those of any previous request.
Exactly one NTS Cookie extension field which MUST be authenticated
and MUST NOT be encrypted. The cookie MUST be one which has been
previously provided to the client; either from the key exchange
server during the NTS-KE handshake or from the NTP server in
response to a previous NTS-protected NTP request. To protect the
client's privacy, the same cookie SHOULD NOT be included in multiple
requests. If the client does not have any cookies that it has not
already sent, it SHOULD initiate a re-run the NTS-KE protocol.
Exactly one NTS Authenticator and Encrypted Extension Fields
extension field, generated using an AEAD Algorithm and C2S key
established through NTS-KE.
The client MAY include one or more NTS Cookie Placeholder extension
fields which MUST be authenticated and MAY be encrypted. The number of
NTS Cookie Placeholder extension fields that the client includes
SHOULD be such that if the client includes N placeholders and the server
sends back N+1 cookies, the number of unused cookies stored by the
client will come to eight. The client SHOULD NOT include more than seven
NTS Cookie Placeholder extension fields in a request. When both the
client and server adhere to all cookie-management guidance provided in
this memo, the number of placeholder extension fields will equal the
number of dropped packets since the last successful volley.
|
| - Transmit time request packet |
+-----------------+---------------------+
|
|
Server -----------+---------------+-----+----------------------->
^ \
/ \
Time request / \ Time response
(mode 3) / \ (mode 4)
/ \
/ V
Client -----+---------------------------------+---------------->
| |
| |
| |
+-----------+----------------------+ +------+-----------------+
|- Generate time request message | |- Verify time response |
| - Include NTPv4 Extension fields | | message |
| o Unique Identifier EF | |- Extract cookie(s) |
| o NTS Cookie EF | |- Time synchronization |
| o | | processing |
| | +------------------------+
|- Generate AEAD tag of NTP message|
|- Add NTS Authentication and |
| Encrypted Extension Fields EF |
|- Transmit time request packet |
+----------------------------------+]]>
The client MAY include additional (non-NTS-related) extension fields
which MAY appear prior to the NTS Authenticator and Encrypted Extension
Fields extension fields (therefore authenticated but not encrypted),
within it (therefore encrypted and authenticated), or after it
(therefore neither encrypted nor authenticated). In general, however,
the server MUST discard any unauthenticated extension fields and process
the packet as though they were not present. Servers MAY implement
exceptions to this requirement for particular extension fields if their
specification explicitly provides for such.
Upon receiving an NTS-protected request, the server SHALL (through some
implementation-defined mechanism) use the cookie to recover the AEAD
Algorithm, C2S key, and S2C key associated with the request, and then
use the C2S key to authenticate the packet and decrypt the ciphertext.
If the cookie is valid and authentication and decryption succeed, the
server SHALL include the following extension fields in its response:
Exactly one Unique Identifier extension field which MUST be
authenticated, MUST NOT be encrypted, and whose contents SHALL echo
those provided by the client.
Exactly one NTS Authenticator and Encrypted Extension Fields
extension field, generated using the AEAD algorithm and S2C key
recovered from the cookie provided by the client.
One or more NTS Cookie extension fields which MUST be authenticated
and encrypted. The number of NTS Cookie extension fields included
SHOULD be equal to, and MUST NOT exceed, one plus the number of
valid NTS Cookie Placeholder extension fields included in the
request. The cookies returned in those fields MUST be valid for use
with the NTP server that sent them. They MAY be valid for other NTP
servers as well, but there is no way for the server to indicate
this.
We emphasize the contrast that NTS Cookie extension fields MUST NOT be
encrypted when sent from client to server, but MUST be encrypted from
sent from server to client. The former is necessary in order for the
server to be able to recover the C2S and S2C keys, while the latter is
necessary to satisfy the unlinkability goals discussed in . We emphasize also that "encrypted"
means encapsulated within the the NTS Authenticator and Encrypted
Extensions extension field. While the body of an NTS Cookie extension
field will generally consist of some sort of AEAD output (regardless of
whether the recommendations of are precisely followed),
this is not sufficient to make the extension field
"encrypted".
The server MAY include additional (non-NTS-related) extension fields
which MAY appear prior to the NTS Authenticator and Encrypted Extension
Fields extension field (therefore authenticated but not encrypted),
within it (therefore encrypted and authenticated), or after it
(therefore neither encrypted nor authenticated). In general, however,
the client MUST discard any unauthenticated extension fields and process
the packet as though they were not present. Clients MAY implement
exceptions to this requirement for particular extension fields if their
specification explicitly provides for such.
Upon receiving an NTS-protected response, the client MUST verify that
the Unique Identifier matches that of an outstanding request, and that
the packet is authentic under the S2C key associated with that request.
If either of these checks fails, the packet MUST be discarded without
further processing.
If the server is unable to validate the cookie or authenticate the
request, it SHOULD respond with a Kiss-o'-Death (KoD) packet (see
RFC 5905, Section 7.4) with kiss code
"NTSN", meaning "NTS negative-acknowledgment
(NAK)". It MUST NOT include any NTS Cookie or NTS Authenticator and
Encrypted Extension Fields extension fields.
If the NTP server has previously responded with authentic NTS-protected
NTP packets (i.e., packets containing the NTS Authenticator and
Encrypted Extension Fields extension field), the client MUST verify that
any KoD packets received from the server contain the Unique Identifier
extension field and that the Unique Identifier matches that of an
outstanding request. If this check fails, the packet MUST be discarded
without further processing. If this check passes, the client MUST comply
with RFC 5095, Section 7.4 where required.
A client MAY automatically re-run the NTS-KE protocol upon forced
disassociation from an NTP server. In that case, it MUST be able to
detect and stop looping between the NTS-KE and NTP servers.
Upon reception of the NTS NAK kiss code, the client SHOULD wait until
the next poll for a valid NTS-protected response and if none is
received, initiate a fresh NTS-KE handshake to try to renegotiate new
cookies, AEAD keys, and parameters. If the NTS-KE handshake succeeds,
the client MUST discard all old cookies and parameters and use the new
ones instead. As long as the NTS-KE handshake has not succeeded, the
client SHOULD continue polling the NTP server using the cookies and
parameters it has.
The client MAY reuse cookies in order to prioritize resilience over
unlinkability. Which of the two that should be prioritized in any
particular case is dependent on the application and the user's
preference. describes the privacy
considerations of this in further detail.
To allow for NTP session restart when the NTS-KE server is unavailable
and to reduce NTS-KE server load, the client SHOULD keep at least one
unused but recent cookie, AEAD keys, negotiated AEAD algorithm, and
other necessary parameters on persistent storage. This way, the client
is able to resume the NTP session without performing renewed NTS-KE
negotiation.
This section is non-normative. It gives a suggested way for servers to
construct NTS cookies. All normative requirements are stated in
and .
The role of cookies in NTS is closely analogous to that of session
cookies in TLS. Accordingly, the thematic resemblance of this section to
RFC 5077 is deliberate and the reader
should likewise take heed of its security considerations.
Servers should select an AEAD algorithm which they will use to encrypt
and authenticate cookies. The chosen algorithm should be one such as
AEAD_AES_SIV_CMAC_256 which resists
accidental nonce reuse. It need not be the same as the one that was
negotiated with the client. Servers should randomly generate and store a
master AEAD key `K`. Servers should additionally choose a non-secret,
unique value `I` as key-identifier for `K`.
Servers should periodically (e.g., once daily) generate a new pair (I,K)
and immediately switch to using these values for all newly-generated
cookies. Immediately following each such key rotation, servers should
securely erase any keys generated two or more rotation periods prior.
Servers should continue to accept any cookie generated using keys that
they have not yet erased, even if those keys are no longer current.
Erasing old keys provides for forward secrecy, limiting the scope of
what old information can be stolen if a master key is somehow
compromised. Holding on to a limited number of old keys allows clients
to seamlessly transition from one generation to the next without having
to perform a new NTS-KE handshake.
The need to keep keys synchronized between NTS-KE and NTP servers as
well as across load-balanced clusters can make automatic key rotation
challenging. However, the task can be accomplished without the need for
central key-management infrastructure by using a ratchet, i.e., making
each new key a deterministic, cryptographically pseudo-random function
of its predecessor. A recommended concrete implementation of this
approach is to use HKDF to derive new
keys, using the key's predecessor as Input Keying Material and its key
identifier as a salt.
To form a cookie, servers should first form a plaintext `P` consisting
of the following fields:
The AEAD algorithm negotiated during NTS-KE.The S2C key.The C2S key.
Servers should then generate a nonce `N` uniformly at random, and form
AEAD output `C` by encrypting `P` under key `K` with nonce `N` and no
associated data.
The cookie should consist of the tuple `(I,N,C)`.
To verify and decrypt a cookie provided by the client, first parse it
into its components `I`, `N`, and `C`. Use `I` to look up its decryption
key `K`. If the key whose identifier is `I` has been erased or never
existed, decryption fails; reply with an NTS NAK. Otherwise, attempt to
decrypt and verify ciphertext `C` using key `K` and nonce `N` with no
associated data. If decryption or verification fails, reply with an NTS
NAK. Otherwise, parse out the contents of the resulting plaintext `P` to
obtain the negotiated AEAD algorithm, S2C key, and C2S key.
IANA is requested to allocate the following entry in the
Service Name and Transport Protocol
Port Number Registry:
Service Name: ntskeTransport Protocol: tcpAssignee: IESG <iesg@ietf.org>Contact: IETF Chair <chair@ietf.org>Description: Network Time Security Key ExchangeReference: [[this memo]]Port Number: [[TBD1]], selected by IANA from the User Port
range
[[RFC EDITOR: Replace all instances of [[TBD1]] in this
document with the IANA port assignment.]]
IANA is requested to allocate the following entry in the
TLS Application-Layer Protocol Negotiation
(ALPN) Protocol IDs registry:
Protocol: Network Time Security Key Establishment, version 1
Identification Sequence:
0x6E 0x74 0x73 0x6B 0x65 0x2F 0x31 ("ntske/1")
Reference: [[this memo]],
IANA is requested to allocate the following entry in the
TLS Exporter Labels Registry:
ValueDTLS-OKRecommendedReferenceNoteEXPORTER-network- time-security/1YY[[this memo]],
IANA is requested to allocate the following entry in the
registry of NTP Kiss-o'-Death Codes:
CodeMeaningReferenceNTSNNetwork Time Security (NTS) negative-acknowledgment (NAK)[[this memo]],
IANA is requested to allocate the following entries in the
NTP Extension Field Types registry:
Field TypeMeaningReference[[TBD2]]Unique Identifier[[this memo]],
[[TBD3]]NTS Cookie[[this memo]], [[TBD4]]NTS Cookie Placeholder[[this memo]],
[[TBD5]]NTS Authenticator and Encrypted Extension Fields[[this memo]],
[[RFC EDITOR: Replace all instances of [[TBD2]], [[TBD3]], [[TBD4]], and
[[TBD5]] in this document with the respective IANA assignments.
IANA is requested to create a new registry entitled
"Network Time Security Key Establishment Record Types".
Entries SHALL have the following fields:
Record Type Number (REQUIRED): An integer in the range
0–32767 inclusive.
Description (REQUIRED): A short text description of the purpose of
the field.
Reference (REQUIRED): A reference to a document specifying the
semantics of the record.
The policy for allocation of new entries in this registry SHALL vary
by the Record Type Number, as follows:
0–1023: IETF Review1024–16383: Specification Required16384–32767: Private and Experimental Use
Applications for new entries SHALL specify the contents of the
Description, Set Critical Bit, and Reference fields as well as which
of the above ranges the Record Type Number should be allocated from.
Applicants MAY request a specific Record Type Number and such requests
MAY be granted at the registrar's discretion.
The initial contents of this registry SHALL be as follows:
Record Type NumberDescriptionReference0End of Message[[this memo]], 1NTS Next Protocol Negotiation[[this memo]],
2Error[[this memo]], 3Warning[[this memo]], 4AEAD Algorithm Negotiation[[this memo]], 5New Cookie for NTPv4[[this memo]], 6NTPv4 Server Negotiation[[this memo]], 7NTPv4 Port Negotiation[[this memo]], 16384–32767Reserved for Private & Experimental Use[[this memo]]
IANA is requested to create a new registry entitled
"Network Time Security Next Protocols". Entries SHALL have
the following fields:
Protocol ID (REQUIRED): An integer in the range 0-65535 inclusive,
functioning as an identifier.
Protocol Name (REQUIRED): A short text string naming the protocol
being identified.
Reference (REQUIRED): A reference to a relevant specification
document.
The policy for allocation of new entries in these registries
SHALL vary by their Protocol ID, as follows:
0–1023: IETF Review1024–32767: Specification Required32768–65535: Private and Experimental Use
The initial contents of this registry SHALL be as follows:
Protocol IDProtocol NameReference0Network Time Protocol version 4 (NTPv4)[[this memo]]32768-65535Reserved for Private or Experimental UseReserved by [[this memo]]
IANA is requested to create two new registries entitled
"Network Time Security Error Codes" and
"Network Time Security Warning Codes". Entries in each SHALL
have the following fields:
Number (REQUIRED): An integer in the range 0-65535 inclusiveDescription (REQUIRED): A short text description of the
condition.Reference (REQUIRED): A reference to a relevant specification
document.
The policy for allocation of new entries in these registries SHALL
vary by their Number, as follows:
0–1023: IETF Review1024–32767: Specification Required32768–65535: Private and Experimental Use
The initial contents of the Network Time Security Error Codes Registry
SHALL be as follows:
NumberDescriptionReference0Unrecognized Critical Extension[[this memo]], 1Bad Request[[this memo]], 32768-65535Reserved for Private or Experimental UseReserved by [[this memo]]
The Network Time Security Warning Codes Registry SHALL
initally be empty except for the reserved range, i.e.:
NumberDescriptionReference32768-65535Reserved for Private or Experimental UseReserved by [[this memo]]This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft, and is based on a proposal described in RFC 7942. The
description of implementations in this section is intended to assist the
IETF in its decision processes in progressing drafts to RFCs. Please
note that the listing of any individual implementation here does not
imply endorsement by the IETF. Furthermore, no effort has been spent to
verify the information presented here that was supplied by IETF
contributors. This is not intended as, and must not be construed to be,
a catalog of available implementations or their features. Readers are
advised to note that other implementations may exist.According to RFC 7942, "this will allow reviewers and working groups
to assign due consideration to documents that have the benefit of
running code, which may serve as evidence of valuable experimentation
and feedback that have made the implemented protocols more mature. It is
up to the individual working groups to use this information as they see
fit".Organization: Ostfalia University of Applied ScienceImplementor: Martin LangerMaturity: Proof-of-Concept PrototypeThis implementation was used to verify consistency and to ensure
completeness of this specification. It also demonstrate
interoperability with NTP's client-server mode messages.This implementation covers the complete specification.The code is released under a Apache License 2.0 license. The source code is available at:
https://gitlab.com/MLanger/nts/Contact Martin Langer: mart.langer@ostfalia.deThe implementation was updated 3rd May 2018.Organization: tbdImplementor: Daniel Fox Franke Maturity: Proof-of-Concept PrototypeThis implementation was used to verify consistency and to ensure
completeness of this specification. This implementation provides the client and the server for the
initial TLS handshake and NTS key exchange. It provides the the
client part of the NTS protected NTP messages. Public domain.The source code is available at:
https://github.com/dfoxfranke/nts-hackathonContact Daniel Fox Franke: dfoxfranke@gmail.comThe implementation was updated 16th March 2018.The Interoperability tests distinguished between NTS key
exchange and NTS time exchange messages. For the
NTS key exchange, interoperability between
the two implementations has been verified successfully.
Interoperability of NTS time exchange messages has been verified
successfully for the case that PoC 1 represents the server and
PoC 2 the client.These tests successfully
demonstrate that there are at least two running implementations
of this draft which are able to interoperate.
The introduction of NTS brings with it the introduction of asymmetric
cryptography to NTP. Asymmetric cryptography is necessary for initial
server authentication and AEAD key extraction. Asymmetric
cryptosystems are generally orders of magnitude slower than their
symmetric counterparts. This makes it much harder to build systems
that can serve requests at a rate corresponding to the full line speed
of the network connection. This, in turn, opens up a new possibility
for DDoS attacks on NTP services.
The main protection against these attacks in NTS lies in that the use
of asymmetric cryptosystems is only necessary in the initial NTS-KE
phase of the protocol. Since the protocol design enables separation of
the NTS-KE and NTP servers, a successful DDoS attack on an NTS-KE
server separated from the NTP service it supports will not affect NTP
users that have already performed initial authentication, AEAD key
extraction, and cookie exchange.
NTS users should also consider that they are not fully protected
against DDoS attacks by on-path adversaries. In addition to dropping
packets and attacks such as those described in
, an on-path attacker can send spoofed
kiss-o'-death replies, which are not authenticated, in response to NTP
requests. This could result in significantly increased load on the
NTS-KE server. Implementers have to weigh the user's need for
unlinkability against the added resilience that comes with cookie
reuse in cases of NTS-KE server unavailability.
Certain non-standard and/or deprecated features of the Network Time
Protocol enable clients to send a request to a server which causes the
server to send a response much larger than the request. Servers which
enable these features can be abused in order to amplify traffic volume
in DDoS attacks by sending them a request with a spoofed source IP. In
recent years, attacks of this nature have become an endemic nuisance.
NTS is designed to avoid contributing any further to this problem by
ensuring that NTS-related extension fields included in server
responses will be the same size as the NTS-related extension fields
sent by the client. In particular, this is why the client is required
to send a separate and appropriately padded-out NTS Cookie Placeholder
extension field for every cookie it wants to get back, rather than
being permitted simply to specify a desired quantity.
Due to the RFC 7822 requirement that
extensions be padded and aligned to four-octet boundaries, response
size may still in some cases exceed request size by up to three
octets. This is sufficiently inconsequential that we have declined to
address it.
NTS's security goals are undermined if the client fails to verify that
the X.509 certificate chain presented by the NTS-KE server is valid
and rooted in a trusted certificate authority. RFC 5280 and RFC
6125 specify how such verification is to be performed in
general. However, the expectation that the client does not yet have a
correctly-set system clock at the time of certificate verification
presents difficulties with verifying that the certificate is within
its validity period, i.e., that the current time lies between the
times specified in the certificate's notBefore and notAfter fields. It
may be operationally necessary in some cases for a client to accept a
certificate which appears to be expired or not yet valid. While there
is no perfect solution to this problem, there are several mitigations
the client can implement to make it more difficult for an adversary to
successfully present an expired certificate:
Check whether the system time is in fact unreliable. If the system
clock has previously been synchronized since last boot, then on
operating systems which implement a kernel-based phase-locked-loop
API, a call to ntp_gettime() should show a maximum error less than
NTP_PHASE_MAX. In this case, the clock SHOULD be considered
reliable and certificates can be strictly validated.
Allow the system administrator to specify that certificates should
*always* be strictly validated. Such a configuration is
appropriate on systems which have a battery-backed clock and which
can reasonably prompt the user to manually set an
approximately-correct time if it appears to be needed.
Once the clock has been synchronized, periodically write the
current system time to persistent storage. Do not accept any
certificate whose notAfter field is earlier than the last recorded
time.
Do not process time packets from servers if the time computed from
them falls outside the validity period of the server's
certificate.
Use multiple time sources. The ability to pass off an expired
certificate is only useful to an adversary who has compromised the
corresponding private key. If the adversary has compromised only a
minority of servers, NTP's selection algorithm (RFC 5905 section 11.2.1) will protect the
client from accepting bad time from the adversary-controlled
servers.
In a packet delay attack, an adversary with the ability to act as a
man-in-the-middle delays time synchronization packets between client
and server asymmetrically . Since NTP's
formula for computing time offset relies on the assumption that
network latency is roughly symmetrical, this leads to the client to
compute an inaccurate value . The delay attack
does not reorder or modify the content of the exchanged
synchronization packets. Therefore, cryptographic means do not provide
a feasible way to mitigate this attack. However, the maximum error
that an adversary can introduce is bounded by half of the round trip
delay.
RFC 5905 specifies a parameter called
MAXDIST which denotes the maximum round-trip latency (including not
only the immediate round trip between client and server, but the whole
distance back to the reference clock as reported in the Root Delay
field) that a client will tolerate before concluding that the server
is unsuitable for synchronization. The standard value for MAXDIST is
one second, although some implementations use larger values. Whatever
value a client chooses, the maximum error which can be introduced by a
delay attack is MAXDIST/2.
Usage of multiple time sources, or multiple network paths to a given
time source , may also serve to mitigate delay
attacks if the adversary is in control of only some of the paths.
At various points in NTS, the generation of cryptographically secure
random numbers is required. Whenever this draft specifies the use of
random numbers, cryptographically secure random number generation MUST
be used. RFC 4086 contains guidelines
concerning this topic.
Unlinkability prevents a device from being tracked when it changes
network addresses (e.g. because said device moved between different
networks). In other words, unlinkability thwarts an attacker that
seeks to link a new network address used by a device with a network
address that it was formerly using, because of recognizable data that
the device persistently sends as part of an NTS-secured NTP
association. This is the justification for continually supplying the
client with fresh cookies, so that a cookie never represents
recognizable data in the sense outlined above. NTS's unlinkability objective is merely to not leak any additional
data that could be used to link a device's network address. NTS does
not rectify legacy linkability issues that are already present in NTP.
Thus, a client that requires unlinkability must also minimize
information transmitted in a client query (mode 3) packet as described
in the draft .
The unlinkability objective only holds for time synchronization
traffic, as opposed to key exchange traffic. This implies that it
cannot be guaranteed for devices that function not only as time
clients, but also as time servers (because the latter can be
externally triggered to send authentication data). It should also be noted that it could be possible to link devices
that operate as time servers from their time synchronization traffic,
using information exposed in (mode 4) server response packets (e.g.
reference ID, reference time, stratum, poll). Also, devices that
respond to NTP control queries could be linked using the information
revealed by control queries. Note that the unlinkability objective does not prevent a client device
to be tracked by its time servers.
NTS does not protect the confidentiality of information in
NTP's header fields. When clients implement , client packet
headers do not contain any information which the client
could conceivably wish to keep secret: one field is random,
and all others are fixed. Information in server packet
headers is likewise public: the origin timestamp is copied
from the client's (random) transmit timestamp, and all other
fields are set the same regardless of the identity of the
client making the request.
Future extension fields could hypothetically contain
sensitive information, in which case NTS provides a
mechanism for encrypting them.
The authors would like to thank Richard Barnes, Steven
Bellovin, Patrik Fältström (Faltstrom), Scott Fluhrer,
Sharon Goldberg, Russ Housley, Martin Langer, Miroslav Lichvar,
Aanchal Malhotra, Dave Mills, Danny Mayer, Karen O'Donoghue,
Eric K. Rescorla, Stephen Roettger, Kurt Roeckx, Kyle Rose, Rich
Salz, Brian Sniffen, Susan Sons, Douglas Stebila, Harlan Stenn,
Joachim Strömbergsson (Strombergsson), Martin Thomson, and
Richard Welty for contributions to this document and comments on
the design of NTS.A game theoretic analysis of delay attacks against time
synchronization protocolsMulti-path Time ProtocolsAuthenticated Encryption
with Associated DataApplication-Layer Protocol
NegotiationClient-to-serverDistributed Denial-of-ServiceExtension FieldHashed Message
Authentication Code-based Key Derivation FunctionInternet Assigned Numbers AuthorityInternet ProtocolKiss-o'-DeathNetwork Time Protocol
Network Time SecurityNetwork Time Security Key ExchangeServer-to-clientSignaling Cipher Suite
ValueTransmission Control
ProtocolTransport Layer
SecurityUser Datagram Protocol